In wafer-to-wafer bonding processes at low temperatures, activation of the wafer surfaces is usually recommended. A plasma is usually applied to activate the wafer surfaces in preparation for wafer-to-wafer bonding at temperatures lower than 100° C. Wafer surface activation via a plasma reduces the processing temperature and increases the wafer bonding strength. The wafer bonding strength is increased due to the increased amount of Si—OH groups, removal of wafer surface-contaminants, enhancement of viscous flow of the surface layer and enhancement of the diffusivity of water and gas trapped at the interface. In particular, atmospheric pressure plasma activated bonding involves igniting a plasma without using a low pressure environment, while maintaining a low temperature.
The temperature of the plasma gas exiting the plasma source is usually around 120° C., which is hotter than desired. Furthermore, safety specifications require the temperature of the outer surface of the plasma source to be less than 60° C. Accordingly, there is a need for improved plasma sources that provide plasma head temperatures lower than 60° C.